1. Field of the Invention
The present invention relates to a zoom lens which can be used in a digital camera, a video camera, a silver film camera, and the like and a camera and a portable information terminal which use the zoom lens.
2. Description of the Prior Art
In recent years, a digital camera is rapidly spread, and there are wide-ranging needs of users for the digital camera. In particular the user always desires high image quality and miniaturization, and high performance and the miniaturization are also desired for the zoom lens used as a taking lens.
Though there are many ideas to use various kinds of zoom lenses for the digital camera, a type suitable for the miniaturization has a construction, in which a first group of lenses having negative focal length, a second group of lenses having positive focal length, and a third group of lenses having the positive focal length are provided in order from an object side, an iris moving integrally with the second group of lenses toward the object side of the second group of lenses is provided, and the second group of lenses moves monotonically from an image side to the object side and the first group of lenses moves so as to correct shift in the position of the image surface, which is attended with the change in the magnification, when the magnification is changed from an end of short focus to the end of long focus. Such construction is described in Japanese Patent Laid-Open No. 039214/1998, Japanese Patent Laid-Open No. 287953/1999, and Japanese Patent Laid-Open No. 2001-033701.
The construction described in Japanese Patent Laid-Open No. 039214/1998 is one of the earliest applications of the above-described type, the basic construction has been completely disclosed, however, it is not sufficient from the viewpoint of the miniaturization and there is scope of improvement.
The construction, in which the construction of Japanese Patent Laid-Open No. 039214/1998 is improved and simplification is advanced, is one described in Japanese Patent Laid-Open No. 287953/1999 or the like. However, since embodiments described in Japanese Patent Laid-Open No. 287953/1999 have only one surface of aspheric surface in the first group of lenses, it is not said that a contrivance for further miniaturization is sufficiently performed.
On the other hand, in the embodiments described in Japanese Patent Laid-Open No. 2001-033701, though the surface of both sides of an air lens formed in the first group is formed to be the aspheric surface, since plastic having a low refractive index is used as a lens material, sufficient correction of aberrations is not performed.
In the construction described in Japanese Patent Laid-Open No. 039214/1998, a taking angle of view of a wide-angle end is about 72xc2x0 and it is not said that the angle of view is sufficiently wide.
It is a first object of the invention to provide a zoom lens which can be further miniaturized.
It is a second object of the invention to provide a high-performance zoom lens which can realize resolution adaptable to an image sensor having three million pixels to five million pixels while the zoom lens is a compact size and the wide angle of view.
It is a third object of the invention to provide a compact and high image quality camera and portable information terminal in which the zoom lens which can be further miniaturized is used as a taking optical system.
A feature of the invention is a zoom lens, which includes a first group of lenses having negative focal length, a second group of lenses having positive focal length, and a third group of lenses having the positive focal length in order from the object side, and in which the second group of lenses moves monotonically from an image side to the object side and the first group of lenses moves so as to correct shift in a position of an image surface, which is attended with a change in magnification, when the magnification is changed from an end of short focus to the end of long focus, the zoom lens is characterized in that the first group of lenses has at least two lenses and an air lens formed between the two lenses, both sides of the air lens are the aspheric surface, and the first group of lenses satisfies the following conditional expressions:
no greater than 1.50
and
ni greater than 1.60,
where no indicates a refractive index to d line of the lens arranged on the object side in the air lens and ni indicates the refractive index to the d line of the lens arranged on the image side in the air lens.
Another feature of the invention is a zoom lens, which includes a first group of lenses having negative focal length, a second group of lenses having positive focal length, and a third group of lenses having the positive focal length in order from the object side, and in which the second group of lenses moves monotonically from an image side to the object side and the first group of lenses moves so as to correct shift in a position of an image surface, which is attended with a change in magnification, when the magnification is changed from an end of short focus to the end of long focus, the zoom lens is characterized in that the second group of lenses includes a positive lens, a negative meniscus lens whose convex surface faces the object side, a positive meniscus lens whose convex surface faces the object side, and a positive lens in order from the object side.
Still another feature of the invention is a zoom lens, which includes a first group of lenses having negative focal length, a second group of lenses having positive focal length, a third group of lenses having the positive focal length in order from the object side, and an iris moving integrally with the second group of lenses toward the object side of the second group of lenses, and in which the second group of lenses moves monotonically from an image side to the object side and the first group of lenses moves so as to correct shift in a position of an image surface, which is attended with a change in magnification, when the magnification is changed from an end of short focus to the end of long focus, the zoom lens is characterized in that the second group of lenses has a three-element cemented lens including a negative lens, a positive lens, and the negative lens in order from the object side.